1. Field of the Invention
The present invention relates to an improved vehicle antenna system which can efficiently detect broadcast waves received by the vehicle body and transmit the detected waves to various onboard receivers.
2. Description of the Prior Art
A pole type antenna is known as one of the conventional vehicle antenna systems. The pole antenna projects exteriorly from the vehicle body and exhibits a favorable reception performance in its own way. However, the pole antenna was always an obstruction the design of vehicle body.
The pole antenna also is disadvantageous in that it may accidentally or intentionally be subjected to damage and in that the pole antenna may produce unpleasant noises during vehicle running at high speeds. Therefore, it is desirable to eliminate the pole antenna from the vehicle body.
Recently, the number of frequency bands, of broadcast or communication waves to be received on automobiles is being increased. If a plurality of pole antennas are located on a vehicle body matching the increased number of frequency bands, the aesthetic appearance of the vehicle is degraded. Furthermore, electrical interference between the pole antennas remarkably degrades the reception performance.
Some attempts have been made to eliminate or conceal pole antennas. One such attempt is that an antenna wire is applied to a rear window glass on a vehicle body.
Another proposal has been made to detect surface currents induced on the vehicle body by broadcast waves. Although it appears that such a proposal apparently provides the most positive and efficient method of utilizing the surface currents flowing on the vehicle body, many actual experiments showed that the method failed, contrary to the above expectation.
One main reason why the surface currents on the vehicle body could not be utilized is that the level of the surface currents is not as large as expected. The prior art mainly intended to detect surface currents flowing on the roof panel of the vehicle body. However, the surface currents on the roof panel could not reach a level sufficient to be utilized in the vehicle antenna system.
The second reason is that the surface currents include noises. The noises are created mainly from the engine ignition system and the battery charging regulator system and therefore cannot be eliminated when the engine is running.
There have been some proposals to overcome such problems. For example, Japanese Patent Publication Sho 53-22418 discloses a vehicle antenna system for utilizing surface currents induced on the vehicle body by broadcast waves, in which an electrical insulator is formed on the vehicle body at a location where the surface currents flow concentratedly. The currents are detected directly by a sensor between the opposite ends of the electrical insulator. It is sure that such an arrangement can detect practicable signals being superior in S/N ratio. However, the vehicle body must partially be cut away to form a space for housing the antenna construction. This is not acceptable in the mass-production of automobiles.
Japanese Utility Model Publication Sho 53-34826 discloses an antenna system comprising a pickup coil for detecting currents on the pillar of the vehicle body. Such a proposal is certainly advantageous in that the antenna can completely be housed within the vehicle body. However, it is not practical since the pickup coil has to be disposed near the pillar in the direction perpendicular to the length of the pillar. Furthermore, such an arrangement cannot provide practicable antenna outputs and appears to be merely an impractical idea.
As described above, the prior art was not necessarily successful in providing an antenna system which detects currents induced on the vehicle body by broadcast waves.
Particularly, the prior art could not provide a proper pickup construction for efficiently detecting currents induced on the vehicle body by broadcast waves and a proper pickup arrangement for obtaining practicable S/N ratios. Rather, many experiments suggested that the antenna system utilizing the surface currents on the vehicle body could not be accomplished.